The present invention generally relates to a technique for inhibiting formation of biofilm or microorganic slime in equipment or facility of a water system where microorganisms exist, such as biofilm on the membrane surface which cause membrane biofouling during the operation of a membrane bioreactor (MBR) for advanced wastewater treatment. More specifically, the present invention relates to a technique for operating a MBR process stably and efficiently without inactivation and loss of enzymes using a specific magnetic carrier that comprises an enzyme for inhibiting biofilm formation (e.g., enzyme for quenching quorum sensing) immobilized thereon.
Recently, a MBR process which is a new type of wastewater treatment process combining a membrane separation process with a biological wastewater treatment reactor (bioreactor) has been actively researched as an alternative to conventional physical/chemical or biological wastewater treatment process, and applied in an actual process. Since the MBR process requires less space in comparison with the conventional physical/chemical or biological wastewater treatment process, the MBR process is more economical. Moreover, the MBR process can be operated stably and efficiently even under sudden external load so as to secure high quality of treated water and to reduce sludge production. Currently, there are about 4,000 plants for operating the MBR process around the world. The market size is about US $216,600,000 in 2005, and the average growth rate is expected to be 10.9% annually until 2010 (The Market Survey Report in 2005, Business Communication Company, Inc.). Therefore, the distribution and research for MBR process have been made actively.
However, during the operation of the MBR process, microorganisms such as bacteria, mold and algae that exist in the MBR start to attach and grow on the membrane surface (“attached growth”), and a film with a thickness of several tens of micrometers as shown in FIG. 1, that is, a biofilm is formed to cover the membrane surface. The biofilm is one of the major causes of membrane biofouling, which serves as filtration resistance to degrade the filtration performance of the membrane such as reduction of the permeability, shortening of the cleaning cycle and lifespan of the membrane, and increase of energy consumption required in filtration, thereby deteriorating the economic efficiency.
In an attempt to solve the above-described problems, various research studies have been done in the past 20 years. However, biofilm formed naturally by microorganisms on the surface of membrane could not be completely removed by conventional physical (e.g., aeration) and chemical means (e.g., coagulation by injection of polymeric coagulant), so that a satisfactory solution related to prevention and control of the biofouling has not been suggested. The outstanding biofouling problem in the MBR process is attributed to the lack of technical consideration and understanding of features of microorganisms in the reactor that directly and indirectly affect biofouling.
It is not easy to remove the biofilm which is a major factor of biofouling of the MBR because the biofilm has high resistance even in external physical and chemical shocks. As a result, although several techniques for inhibiting biofouling by the conventional physical and chemical methods are effective in the initial stage of the biofilm formation, the effect for inhibiting biofouling after maturation of the biofilm becomes degraded. In order to overcome the problem of conventional methods, new technology has been required to regulate and control characteristics of microorganisms in the reactor, specifically formation and growth of biofilm on the membrane surface.
In addition to the above MBR process, biofilm or microorganic slime is formed on a material surface by microorganisms existing in water systems (“water system” is used herein to mean equipment, facilities and process etc. using water or holding water.) such as a water tank and a water pipe of buildings and industrial facilities, thereby degrading performance of the equipment (e.g., corrosion of metal surfaces, degradation of efficiency of cooling towers, and contamination of water pipe networks by microorganisms) and deteriorating external appearance. As a result, although removal of biofilm or microorganic slime is required, there have been no solutions based on research on characteristics of microorganisms other than conventional physical/chemical methods.
Meanwhile, microorganisms tend to react to environmental change such as temperature, pH, and nutrients, to synthesize specific signal molecules and exhaust/absorb the molecules outside cells, thereby perceiving the peripheral cell density. When the cell density increases so that the concentration of the signal molecules reaches a threshold level, a specific gene expression starts. As a result, microbial group behavior is regulated, which is called a “quorum sensing phenomenon.” Generally, the quorum sensing phenomenon occurs under conditions with high cell density. As examples for this phenomenon, symbiosis, virulence, competition, conjugation, antibiotic production, mobility, sporulation, biofilm formation have been reported (Fuqua et al., Ann. Rev. Microbiol., 2001, Vol. 50, pp. 727˜-751).
Specifically, the quorum sensing mechanism of microorganisms in the MBR process may occur more frequently and easily in the case of a biofilm state with a higher cell density than in the case of a suspended growth state with a lower cell density. Since Davies et al. reported in 1998 that the quorum sensing mechanism of Pseudomonas aeruginosa, which is an opportunistic pathogen, has a close relation to various features of biofilm such as the extent of its formation, the physical structure of it such as thickness and morphology, and the antibiotic resistance of the microorganism (Science, Vol. 280, pp. 295˜298), research for inhibiting biofilm formation by artificial regulation of the quorum sensing mechanism has been made so as to prevent contamination of medical appliances (Baveja et al., Biomaterials, 2004, Vol. 50, pp. 5003˜-5012) and catch of a plant diseases (Dong et al., Nature, 2001, Vol. 411, pp. 813˜-817).
A conventional method for inhibiting biofilm formation by regulating the microorganism quorum sensing mechanism is classified into two categories. Firstly, the biofilm formation can be inhibited by injecting an antagonist that has been reported to have a similar structure to a signal molecule used in the quorum sensing mechanism and to be competitive with the signal molecule in a gene expression site. As a representative antagonist, furanone secreted by Delisea pulchra, which is a type of red algae, and its halogenated derivatives have been reported (Henzer et al., EMBO Journal, Vol. 22, 3803˜3815). Secondly, the biofilm formation can be inhibited by an enzyme for decomposing a signal molecule used in the quorum sensing mechanism (enzyme for inhibiting biofilm formation such as an enzyme for quenching microorganism quorum sensing) or by microorganisms for producing the enzyme. For example, Xu et al. developed in 2004 a method for inhibiting biofilm formation in various surfaces by injecting a solution of acylase that is an enzyme for decomposing acyl-homoserine lactone (AHL) which is a signal molecule of Gram-negative bacteria (see U.S. Pat. No. 6,777,223).
The present inventor applied the technologies for regulating the signal molecule of microorganisms to quench the quorum sensing mechanism thereby inhibiting biofilm formation to a water system process like the MBR so as to inhibit biofouling by the formation of biofilm in micrometer scale on the material surface (e.g., membrane surface), thereby improving performance of the water system process (e.g., MBR process).
A simple method for applying the quorum sensing regulation technique to the process of water system like the MBR process involves injecting the solution of antagonist or the enzyme (e.g., enzyme for inhibiting biofilm formation) into a bioreactor with high concentration to prevent biofouling of the MBR. However, this method may cause various economic and technical problems.
First, since the antagonist or the enzyme is a soluble material, that is much smaller size than the pore size of a microfiltration membrane (0.1˜0.45 μm) which is generally used in the MBR process, they tend to slip out of the MBR along with the membrane permeate. Also, while sludge is taken out periodically to keep sludge retention time (SRT) constant during the operation of the MBR, the antagonist and the enzyme may be lost along with the sludge. It is difficult to maintain effective concentration of the antagonist or the enzyme in the reactor of a MBR.
Furthermore, since temperature and pH suitable for activity, and reactive substrate are limited in the case of enzymes, it is probable that the enzyme can be inactivated in a soluble state in the MBR in which various microorganisms exist with high concentration and various contamination materials are flowed.
Because of the problems described above, a large amount of enzymes, which may be costly, should be injected periodically in order to maintain a sufficient activity of enzymes to quench the quorum sensing phenomenon in the MBR. As a result, the operational cost of a large-scale MBR may be remarkably increased.
In the case of equipment for water systems such as a water tank or a water pipe in addition to the MBR described above, when a third substance such as an enzyme with a soluble state is injected to inhibit formation of biofilm or microorganism slime, the above substance should be removed before water is finally used. This may not be economical.
Therefore, a new technique that can solve the mass loss and inactivation of the enzymes described above is required in order to prevent biofouling by using the technique for inhibiting biofilm formation with the enzyme for inhibiting biofilm formation such as the enzyme for quenching quorum sensing in the water system like the MBR process.